This invention relates generally to radio frequency array antenna systems and more particularly to radio frequency array antenna systems adapted to form a plurality of distinct beams of radio frequency energy.
As is known, a radio frequency array antenna system may be arranged to produce a plurality of distinct, spaced beams of radio frequency energy. Typically, each one of the plurality of beams has the gain and beamwidth of the entire antenna array and a different scan angle with respect to the boresight axis of the array. As is also known, such plurality of spaced beams may be produced by coupling each array antenna element through a different partially constrained electrical path to a corresponding plurality of beam ports, the partially constrained electrical paths comprising an electro-magnetic lens which equalizes the time delay of the electro-magnetic energy between the beam ports and all points on corresponding planar wavefronts of either transmitted or received energy. One such antenna system is described in U.S. Pat. No. 3,761,936, entitled, "MultiBeam Array Antenna", inventors D. H. Archer et al, issued Sept. 25, 1973, and assigned to the present assignee.
While such an array antenna system has proved satisfactory in some applications, it is often desirable that an array antenna having relatively high effective radiated power (ERP) for transmitted radio frequency energy and correspondingly high sensitivity to received energy. One conventional way of achieving such high ERP and sensitivity is to increase the size of the array by adding antenna elements thereto, thereby enlarging the antenna aperture and increasing the gain of the array. This typically requires the electromagnetic lens feeding the array to be enlarged to accommodate the additional antenna elements. As discussed in the above-referenced patent, the electromagnetic lens is typically fabricated as a stripline, parallel plate lens, with a printed circuit defining the partially constrained electrical paths being formed on one side of a dielectric substrate and a metallic ground plane being formed on the other side thereof. A second dielectric slab is disposed over the printed circuit with a second metallic ground plane covering the exposed side of such dielectric slab. The printed circuit is typically relatively thin due to the high frequencies of the transmitted and received radio frequency energy. Thus, the larger required lens is fragile and difficult to manufacture. Also, each dielectric slab of such a large lens must often comprise several sections of dielectric material, the performance of such "sectioned" lens being degraded over that of a lens fabricated from a single section of dielectric material. In applications wherein the array antenna is disposed in a housing for mobile use, increasing the size of the electromagnetic lens necessitates a larger housing, which may be unacceptable where the size and weight of the system must be kept small. The size of the electromagnetic lens could be reduced by increasing the dielectric constant of the dielectric material, but such lens would be difficult to fabricate because the array ports thereof would be disposed closer together by decreasing the size of the lens.
One possible solution to the problems encountered with a single large lens would be to implement the lens as a pair of modular, identically constructed lenses, each lens being one-half the size of the single large lens. The array ports of each lens would feed one-half of the array of antenna elements. The pair of lenses and sub-arrays of antenna elements thus would form corresponding pairs of overlaying beams of energy associated with each beam port thereof. Each pair of overlaying beams produced by the two halves (i.e., subarrays) of the array would spatially combine to produce a composite beam having a width one-half that of each one of the pair of constituent beams. As is known, at the upper end of the operating frequency band of the array antenna, it is desired that adjacent composite beams cross over one another at the -3dB points thereof to ensure that the sector (e.g. azimuth) scanned by the antenna is covered relatively uniformly by the composite beams produced thereby. However, the half-width composite beams which would be produced by the modular pair of identical lenses would have crossovers at -12dB, thereby providing "holes" in the coverage provided by the array antenna. Additional beam positions could be provided to re-establish the desired -3dB crossovers, but such would require doubling the number of beams, thereby necessitating twice as many beam ports on each lens. Also, since the beams produced by the array conventionally are steered across the azimuth of the array by switches which sequentially switch each lens beam port, doubling the number of beam ports on each lens would require doubling the number of throws of each azimuth beam-steering switch, thus increasing the complexity of the antenna system.